There is a significant push to develop commercial forms of a hydrometallurgical process to recover various types of metal from ore bodies. The significant advantage of a hydrometallurgical process over the standard smelting process, is the significant reduction in sulfur dioxide emissions. Although the chemistry might appear to be relatively direct in extracting, for example, copper and zinc from sulfide ores, all known commercial approaches in this regard have either failed or are not economically viable. It is known that several of these hydrometallurgical processes for leaching copper, zinc and the like from either ore concentrate or a rich ore involve the use of sulfuric acid and/or nitric acid and/or nitrate salts.
U.S. Pat. No. 3,888,748 discloses a metal recovery process whereby copper may be recovered from sulfide ore concentrates containing minerals such as chalcopyrite (CuFeS.sub.2). The copper is recovered by contacting the ore concentrate with a dilute aqueous solution of nitric acid and sulfuric acid to give a leachate containing in solution of copper salts and iron salts and a residue. The leachate is subjected to further processing in which copper is recovered and iron is precipitated as jarosite. Jarosite has no value and can complicate the recovery process. The nitrate ions and its derivatives must be substantially removed from the leachate to facilitate an electrowinning of copper or zinc from the solution.
In U.S. Pat. No. 3,910,636 a process is disclosed for in-situ mining. Holes are drilled into an ore body and the holes are filled with an acid leaching solution containing nitrate ions at a pH range of 0.2 to about 2.0. However, the solution becomes diluted and hence, the process is relatively slow in leaching copper from the ore. In addition, the process cannot normally be used in limestone formations.
Another in-situ chemical mining process is disclosed in U.S. Pat. No. 3,912,330 which is specifically directed at dealing with copper porphyry ores. Catalytic amounts of nitrate ion are added to an oxygenated sulphuric acid leach medium to improve the rate of copper extraction from copper sulfide ores. The nitrate concentrations may range from 0.05 to 0.50% and the acid media is oxygenated at oxygen pressures from 25 psi to 200 psi. Jarosite is said to be precipitated and the process is acknowledged to be unsuitable for surface heap leaching.
U.S. Pat. No. 4,647,307 teaches that complex copper ores can be treated with oxidizing acid media. Arsenical ores can be processed especially well with this system.
Published literature in the field includes a Ph.D. dissertation (G. Van Weert, Ph.D. Dissertation, De Technische Universiteit, Delft, Holland, 1989) which contains an Appendix giving a summary of treatments for complex ores and concentrates containing chalcopyrite. Avramides et al (Hydrometallurgy, 5, 325-36 (1980)) describe a process in which the chalcopyrite leaching process consists of leaching with acetonitrile solutions of cupric and cuprous ions. Kiknadze et al (Izv. Akad. Nauk Gruz. SSR, Set. Khim., 6 363-6 (1980)) describe a ferric ion leach of chalcopyrite where the ferric ion is regenerated with chlorine. Another ferric ion leach is described by Tkacova and Balaz (Hydrometallurgy, 21 103-12 (1988)) purports to increase the surface area of chalcopyrite but mentions also that the sulfur on the surface retards the dissolution of the chalcopyrite. Pomanianowski et al. (Electrocatal., Mater. Symp. Electrochem. Sect. Pol. Chem. Soc., 9th meeting date 1987, 241-7, Edited by Pawel Nowak, Pol. Chem. Soc.: Warsaw Pol.) found that deposition of minor amounts of silver on the surface of chalcopyrite catalysed the rate of dissolution by electrochemical means.
The above processes however are inadequate from one or more of the following perspectives:
i) the processing cost is uneconomical relative to the value of the metals recovered, PA1 ii) inoperable in a commercial scale, PA1 iii) polluting off gases, PA1 iv) inefficient recovery of the valuable metal(s), PA1 v) off gases cannot be treated for recycle and re-use in the process, PA1 vi) processing conditions require the use of pressurized reactors to obtain conversions of copper and zinc sulfides to corresponding sulfate salts. PA1 i) contacting the ore concentrate derived copper and zinc sulfides with sulfuric acid and with nitric acid to form a reaction mixture in an acidic solution, PA1 ii) maintaining the reaction mixture at a temperature in the range of 110.degree. C. to 170.degree. C. while continuously mixing the reaction mixture, PA1 iii) adding sufficient sulfuric acid and nitric acid to the reaction mixture to form a light coloured precipitate and a dark coloured precipitate in the reaction mixture, the light precipitate comprising water soluble sulfate salts of copper sulfate and zinc sulfate and the dark precipitate being water insoluble and comprising sulfur and gangue, PA1 iv) introducing a source of oxygen to the reaction mixture to promote oxidation in the presence of the nitric acid, of the sulfides to sulfates and to oxidize gaseous NO.sub.x reaction products to regenerate nitric acid for use in the reaction mixture, PA1 v) removing the light and dark precipitates and any entrained acidic solution from the reaction mixture, PA1 vi) separating the light and dark precipitate from the acidic solution in preparation for treatment of the light precipitate for recovery of copper sulfate and zinc sulfate from the light precipitate, and PA1 vii) recycling the acidic solution to the reaction mixture.
The process according to this invention overcomes several of the above problems in providing a process which does not have to operate under pressurized conditions. Because of the use of high concentrations of sulfuric acid in the presence of oxidizing nitric acid and oxygen gas, the desired metals are recovered as water soluble salts precipitated in the acidic solution of the reaction mixture which is operated at temperatures in the range of 110.degree. C. to 170.degree. C. at which temperature and high acidity the water soluble metal salts are less soluble.